Tanana Valley Mirages
Art Greenwelt of Fairbanks asks the question: "Why do peaks of the Alaska Range, when seen from Fairbanks during the winter, sometimes appear to be chopped off, flattened into mesas, or even floating in midair?"
To answer the question, it is first necessary to explain that light rays always bend toward a medium of higher density. This is why a stick placed in water appears to be sharply bent at the air-water interface.
The same thing happens in the atmosphere, but rather than an abrupt interface between two dissimilar materials (water and air), the transition is more gradual.
Cold air is more dense than warm air; hence, light rays are always bent in the direction of the colder air layers. Under normal conditions, air temperature falls off gradually and uniformly with increasing altitude, and the curvature of light rays is so slight that it is not readily apparent -- things appear to the eye essentially as they are.
When layering of hot and cold air occurs, however, light rays are more strongly bent, often producing astonishing effects. The desert mirage, so popular in novels of the old west, is a result of a layer of heated air overlying the surface, and a rapid upward cooling. Light rays between two points are bent sharply upward (i.e., convex downward) causing objects to appear lower than they are and often creating a shimmery illusion of water in the distance. There is a story told by residents of Cochise County in Arizona of the pilot who attempted to land his float plane on such a lake, only to discover, too late, that it didn't exist. The results were spectacular.
The Arctic mirage is a different story altogether. During inversion conditions in the Tanana Valley, there is a layer of dense, cold air next to the ground with warmer air overlying it. Under these conditions light rays between two points are bent downward (convex upward), making it appear that distant objects are higher than they actually are. This produces the floating peaks of the Alaska Range that are sometimes observed.
The most spectacular mirage that can be seen from Fairbanks is called the Fata Morgana. This apparition takes its name from the Italian fable about the magic undersea palace of the fairy of that name. It occurs during periods of inversion that also include alternating layers of warm and cold air.
During an episode of Fata Morgana, the light paths are quite complicated. All imaginable mirages may be combined, with upright, inverted, enlarged, diminished and distorted multiple images appearing.
While this beautiful effect exceeds all other types of mirage in variety, like the others the Fata Morgana results from the natural laws that govern the behavior of light waves traveling through the air.
